Collisions of droplets on spherical particles

Georgios Charalampous,Yannis Hardalupas

doi:10.1063/1.5005124

Abstract

Head-on collisions between droplets and spherical particles are examined for water droplets in the diameter range between 170 μm and 280 μm and spherical particles in the diameter range between 500 μm and 2000 μm. The droplet velocities range between 6 m/s and 11 m/s, while the spherical particles are fixed in space. The Weber and Ohnesorge numbers and ratio of droplet to particle diameter were between 92 &lt; We &lt; 1015, 0.0070 &lt; Oh &lt; 0.0089, and 0.09 &lt; Ω &lt; 0.55, respectively. The droplet-particle collisions are first quantified in terms of the outcome. In addition to the conventional deposition and splashing regimes, a regime is observed in the intermediate region, where the droplet forms a stable crown, which does not breakup but propagates along the particle surface and passes around the particle. This regime is prevalent when the droplets collide on small particles. The characteristics of the collision at the onset of rim instability are also described in terms of the location of the film on the particle surface and the orientation and length of the ejected crown. Proper orthogonal decomposition identified that the first 2 modes are enough to capture the overall morphology of the crown at the splashing threshold.

Highlights

The spray drying process is widely used to convert liquid slurry to dry particles for the manufacture of a range of products, including foodstuffs, like instant coffee and powdered milk, household detergents, pharmaceuticals, or carbide particles for the manufacture of cutting tools (Masters, 1991)
D = Droplet diameter D0 = Particle diameter h = Crown height Oh = Ohnesorge number Re = Reynolds number, based on D s = Arc of film spread on the particle surface U = Droplet velocity We = Weber number, based on D Ω = Droplet to particle diameter ratio φ = Angle between the vertical direction and the crown base μ = Liquid viscosity θ = Ejection angle between the crown and the horizontal direction ρ = Liquid density σ = Surface tension ζ = Deflection angle between the crown and the normal direction to the particle surface
The direction of the crown in relation to the target surface is important, as it is the direction at which the jets will begin to emerge from the crown rim. This direction becomes progressively tangential to the horizontal as the droplet-particle diameter ratio increases. This shows that the interaction of the liquid droplet and the film on the particle surface changes with Ω, and Ω needs to be taken into account in the analytical examination of the kinematic discontinuity

Summary

INTRODUCTION

The spray drying process is widely used to convert liquid slurry to dry particles for the manufacture of a range of products, including foodstuffs, like instant coffee and powdered milk, household detergents, pharmaceuticals, or carbide particles for the manufacture of cutting tools (Masters, 1991). Depending on the spray dryer operation, for example, in a swirling flow configuration, the rate of collisions can be significant (Huntington, 2004) This can alter the final powder size and, as a result, the process can become less efficient since the probability of powder particles that fall outside the desired size range increases. The impingement outcomes in this case can be rebounding, deposition, prompt splash, and crown splash, which are similar to those described for impact on dry surfaces. When droplets collide with solid particles, the impact surface is rigid, curved, and of finite extent This paper closes with a summary of the main conclusions

EXPERIMENTAL ARRANGEMENT AND METHODS

Collision outcomes

Mapping of the droplet-particle collision regimes

Onset of instability on the crown rim

Proper orthogonal decomposition of droplet-particle collisions

CONCLUSIONS